Sweltering temperatures due to climate change are prompting an increasing number of people around the world to plug in air-conditioners. Emissions from the power needed to run these systems, as well as the refrigerants they use, are further heating the planet.

For an energy-free route to cooling, researchers report a way to cool the inside of well-ventilated buildings with the help of recently developed “radiative cooling” materials and some simple building design changes. In tests, they were able to maintain an interior temperature of almost 4°C below the outside temperature, despite the inside being actively heated from within. The results appear in the journal Cell Reports Physical Science.

Radiative cooling materials, first developed six years ago by researchers at Stanford University, work by reflecting sunlight like a mirror, and by emitting heat at a wavelength that passes through the Earth’s atmosphere directly into space. Used as coatings or panels on rooftops, they can help cool down buildings on hot, sunny days.

These materials work by keeping exterior roof surfaces cooler, which prevents buildings from overheating. “However, true cooling entails removing heat, which is more challenging than improving heat protection,” McGill University architecture professor Salmaan Craig and his colleagues write.

Radiative cooling materials work well on hot days when the interior temperature is warmer than the outside, but when buildings are cooled, insulation limits the flow of heat to the outside. “Interior heat rejection is even more challenging in the aftermath of the COVID-19 pandemic now that healthy ventilation is a priority,” they write, because buildings let in warm air from the outside multiple times a day.

The McGill team wanted to test if it would be possible to cool a well-ventilated building below the outside temperature with the help of simple architectural changes. So they installed two experimental boxes under the open sky in the warm, dry climate of the Topanga Valley in California. A set of water bottles served as the experimental masses that needed to be cooled. The first reference box contained the water bottles, a heat source, and two vent pipes on top. The second test box also had the bottles and heat source, but the vent pipes were at the bottom, and the roof was an uninsulated aluminum plate with a radiative cooling material glued on the top surface.

The tests showed that in the reference box, ventilation only occurred at night when the inside was warmer than the exterior, which created an upward flow of air through the shortest vent. During the day, there is no ventilation since the interior is cooler than the exterior. In the test box, by contrast, there is no ventilation at night because the vents are at the bottom of the box; only the roof helps cool the interior air during the night. But ventilation happens during the day because the interior is cooler than the exterior.

As a result, the test box interior stayed about 4°C cooler than outside temperatures and almost 9°C below the reference box, despite being actively heated from within and venting seven air changes per hour during the day.

“We hope that materials scientists, architects, and engineers will be interested in these results, and that our work will inspire more holistic thinking for how to integrate breakthroughs in radiative cooling materials with simple but effective architectural solutions,” Craig said in a press release.

Source: Remy Fortin et al. Passive radiative cooling to sub-ambient temperatures inside naturally ventilated buildings. Cell Reports Physical Science, 2023.